linux-kernel - [PATCH rcu 04/15] srcu: Bit manipulation changes for additional reader flavor

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Message-Id: <20241015161112.442758-4-paulmck@kernel.org>
Date: Tue, 15 Oct 2024 09:11:01 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: rcu@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	kernel-team@...a.com,
	rostedt@...dmis.org,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Kent Overstreet <kent.overstreet@...ux.dev>,
	bpf@...r.kernel.org
Subject: [PATCH rcu 04/15] srcu: Bit manipulation changes for additional reader flavor

Currently, there are only two flavors of readers, normal and NMI-safe.
Very straightforward state updates suffice to check for erroneous
mixing of reader flavors on a given srcu_struct structure.  This commit
upgrades the checking in preparation for the addition of light-weight
(as in memory-barrier-free) readers.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Cc: Alexei Starovoitov <ast@...nel.org>
Cc: Andrii Nakryiko <andrii@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: <bpf@...r.kernel.org>
---
 kernel/rcu/srcutree.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 18f2eae5e14bd..abe55777c4335 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -462,7 +462,7 @@ static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx)
 		if (IS_ENABLED(CONFIG_PROVE_RCU))
 			mask = mask | READ_ONCE(cpuc->srcu_reader_flavor);
 	}
-	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask >> 1)),
+	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask - 1)),
 		  "Mixed NMI-safe readers for srcu_struct at %ps.\n", ssp);
 	return sum;
 }
@@ -712,8 +712,9 @@ void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor)
 	sdp = raw_cpu_ptr(ssp->sda);
 	old_reader_flavor_mask = READ_ONCE(sdp->srcu_reader_flavor);
 	if (!old_reader_flavor_mask) {
-		WRITE_ONCE(sdp->srcu_reader_flavor, reader_flavor_mask);
-		return;
+		old_reader_flavor_mask = cmpxchg(&sdp->srcu_reader_flavor, 0, reader_flavor_mask);
+		if (!old_reader_flavor_mask)
+			return;
 	}
 	WARN_ONCE(old_reader_flavor_mask != reader_flavor_mask, "CPU %d old state %d new state %d\n", sdp->cpu, old_reader_flavor_mask, reader_flavor_mask);
 }
-- 
2.40.1